In a heated type sensor in which a sensing portion of an oxygen or a NOx sensor is heated by a heater, the characteristics of the sensing portion are dependent on its temperature which will vary due to the variation of temperature of the environment or the heater's own temperature variation. A conventional temperature control method for heater in which the heater temperature is maintained to a predetermined value, for example, 300-400.degree. C. is described in the JP-A-60 114,758 official gazette.
The heated type sensor maintains constant temperature of a detective part (3) with a main heater (30) and an auxiliary heater (32). A leakage current originating in the deterioration of the insulation substrate (2) is absorbed by a guard heater.
The good thermal conductivity (high thermal conductivity) insulation substrate or film (2) contains the detective part that is maintained at the high temperature, for example, 400.degree. C., and main and auxiliary heaters are nearly arranged. Alternatively, a guard heater for the leakage current protection is arranged between the main and auxiliary heaters adjacent to the auxiliary heater. A bridge circuit (18) contains the auxiliary heater in one side, and first, second and third resistors (12, 14, 16) in the remaining three sides. The pyro-control circuit comprises an amplifier or a voltage follower (20) which supplies a voltage to the main heater, and another amplifier (24) having an inverting input connected to a serial node between the first resistor and the auxiliary heater, a non-inverting input connected to another serial node between the second and third resistors, and an output end connected to a control input end of the voltage follower. This amplifier controls the power supply voltage to the bridge circuit and the main heater based on the output of the bridge circuit. The second amplifier (52) controls the voltage of a node (50) with the guard heater to be identical to that of the node (13) with the auxiliary heater, based on an output of the second bridge circuit (44) containing auxiliary heater and guard heater (40). (See FIG. 4).
In the aforementioned temperature control method, a thermometric element is disposed adjacent to the heated type sensor with a space therebetween containing gas or ambient air to measure its environmental temperature, and the temperature of the sensing portion is constantly maintained by controlling a power supply to the heater based on the measurement value. The method necessitates the thermometric element by which the temperature of ambient air should be measured, and complex heated type sensor structure and heat control circuitry resulting in higher cost.
Further, an indirect thermal measurement of the sensing portion cannot result in high accuracy because under the condition of gas or air flow through the space, the temperature of the sensing portion is not precisely transferred to the element, and also a delay of temperature transfer to the element can possibly lead to uncontrollable situation such as temperature convergence or divergence.
As another conventional example without any auxiliary heater, a tin oxide film gas sensor is disclosed in 111 page of Nikkei Electronics issued on Jan. 20, 1992 by Nikkei BP publisher. The gas sensor provides four silicon film substrates projected inwardly from a comer of a center-scooped rectangular silicon dioxide substrate chip, and the tin oxide film and platinum heater are deposited on one side of each of the silicon film substrates. The present inventor proposed some improvement regarding the above method in Japanese Patent Application No. 7-328,395.
FIG. 1 shows an example of a well-known bridge type heat control circuit. The circuit comprises a bridge circuit 18 having, for example, a platinum film heater 10 on one side thereof and the resistors 12, 14 and 16 on the remaining three sides thereof, respectively, an emitter follower 20 which supplies a voltage to the bridge circuit 18, and an amplifier 24 having its inverting input connected to a serial node 13 between the resistor 12 and the heater 10, a non-inverting input connected to another serial node 17 between the resistors 14 and 16, and an output connected to the base of the emitter follower 20 through a resistor 22.
Resistance value of the platinum film heater 10 is changed with temperature as the temperature T--resistor R characteristics of FIG. 2. If electric potential values of the serial nodes 13 and 17 are e1 and e2, respectively, the amplifier 24 and the emitter follower connected transistor are so operated that the electric potential e1 is identical to the electric potential e2, i.e., the resistance ratio of the resistor 12 to the heater 10 is identical to that of the resistors 14 to 16 to maintain the temperature of the heater 10 to a predetermined value.
Then, when the temperature of the heater 10 is lower than the predetermined value, output voltage of the amplifier 24 and emitter follower 20 are increased under the condition of e1&lt;e2 to filter increase the power supply to the heater 10. When the temperature of the heater 10 is higher than the predetermined value, output voltage of the amplifier 24 and emitter follower 20 are decreased under the condition of e1&gt;e2 to decrease the power supply to the heater 10.
As described above, it is obvious that the bridge type heat control circuit shown in FIG. 1 has following problems. In order to detect temperature change, i.e., resistance change of the heater 10 precisely, the resistor 12 having higher resistance value than that of the heater 10 may be used. In that case, however, the heat quantity generated in the resistor 12 is also increased more than that in the heater 10. Thus the energy loss by the resistor 12 is increased, the temperature increase of the resistor 12 simultaneously becomes excessive, and then dangerous.
Since the resistance value of the reference resister 12 changes according to its temperature coefficient, it is difficult to obtain an appropriate heater temperature. Moreover, in case that the resister 12 has a positive temperature coefficient of resistance, the temperature and thus the resistance value of the resister 12 are increased when the heater 10 is heated. Therefore, the settling time to e1=e2 is delayed.